In the field of forensic science, investigations of crimes involving firearms use ballistic comparison test to determine if a bullet or a spent cartridge case found on the crime scene has been fired by a firearm in question. Ballistic comparison tests rely on the fact that when a bullet is fired by a firearm, striations are created on the bullet surface and these striations have enough unique features to represent a signature of the firearm. With regards to spent cartridge cases when a firearm is fired unique identifiable features (marks) are impressed or striated onto various areas of the cartridge case. These unique marks are transferred from the firearm to the cartridge case each time the firearm is discharged; these marks also represent the signature of the firearm. Therefore by comparing the striations or impressed characteristics of two bullets or two cartridge cases, it is possible to conclude if they have been fired by the same firearm.
Most existing automatic ballistic comparison systems use 2D imaging techniques to obtain images of the striations or impressed marks on the ballistic piece of evidence (i.e. the bullet or the cartridge case) under test. They then compare these images to other images from a database of known firearms. The firearm that was used to fire the ballistic piece of evidence (BPOE) under test can be identified when a match is obtained between the images of the BPOE under test and the images of the database corresponding to a BPOE fired by the same firearm.
However, ballistic matching techniques based on 2D-imaging present many drawbacks. In particular, it is found that the technique lack robustness: the images resulting from a 2D imaging are very dependent on the exact way the BPOE is illuminated and imaged as well as on the surface conditions of the BPOE, therefore affecting the performance of the technique.
Recently, Bachrach et al. in their U.S. Pat. No. 6,505,140 B1 have proposed to use a confocal technique, also referred to as a 3D-imaging technique, to better study and identify the features of a bullet surface. A confocal sensor enables one to measure the striation structure and this leads to a more reliable way than a 2D-imaging technique to characterize the striations on a bullet surface. However, the striation characteristics are resolved along particular direction of a bullet striation, to obtain what is called a depth profile of the bullet striation. Therefore, although a 3D-imaging technique is used, no real 3D bullet signature is established and used for further ballistic comparison.